KINETICS OF THE URANIUM DIOXIDE ZIRCALOY REACTIONS AT HIGH-TEMPERATURES

Chemical interactions between uranium dioxide (UO2) fuel and Zircaloy-4 (Zry) cladding material under Power Cooling Mismatch (PCM) conditions are described. The out-of-pile UO2/Zry reaction experiments were performed with short Zircaloy tubular specimens filled with UO2-pellets between 900 and 1500°C under variable UO2/Zry contact pressures and under inert gas conditions. The annealing periods varied between 3 and 60 min. Due to the thermodynamic instability between UO2 and Zry, UO2 is partially reduced by Zry. The extent of UO2/Zry reaction, as well as the composition of phases, depends decisively on whether or not good contact between UO2 and Zry has been established. If solid contact exists, a (U, Zr)-alloy rich in uranium, as well as on oxygen stabilized α-Zr(O) phase are formed. The degree of chemical interaction between UO2 and Zry is determined by oxygen diffusion in Zry and in the α-Zr(O)-phase formed. The UO2/Zry reactions obey a parabolic time law; for the temperature dependence, the corresponding Arrhenius plot is given. Thin oxide layers on the cladding tube inner surface prevent UO2/Zry reactions from taking place below 1100°C. Simulated fission products (cesium, iodine, tellurium) in UO2 exert no influence on the extent of the reaction. © 1979.